Brussels Sprouts Potato Gratin Casserole

Picture a casserole that arrives at the table glowing gold: emerald Brussels sprouts tucked among tender, butter-slicked potato coins, edges crisp and frilled, a Gruyère perfume mingling with sweet onion and nutmeg.

Imagine the first forkful—creamy, softly peppered, a little tang from Dijon—melting into earthy comfort as the top crunches delicately under your bite.

This matters to me because it’s the kind of honest, nourishing food that settles a room: it warms the kitchen, coaxes conversation, and turns a simple night into something to remember.

It’s equally at home on busy weeknights when you need hands-off ease or on slow Sunday suppers when everyone lingers.

One December, after a long drive and an even longer to-do list, this gratin became dinner and next-day lunches, smoothing the edges of the week with every reheated slice.

Ready? Let’s cook!

Why You’ll Love It

• Delivers cozy, creamy comfort with crisp, golden cheesy top
• Balances sweet onions, nutmeg, and Dijon for complex flavor
• Makes an impressive, crowd-pleasing side or vegetarian main
• Preps ahead easily; bake later for stress-free entertaining
• Uses simple ingredients with adaptable cheese and spice options

Ingredients

• 680 g Brussels sprouts, halved — trim ends well (choose firm, bright green heads)
• 680 g Yukon Gold potatoes, thinly sliced — about 1/8-inch thick (pick waxy, smooth skins)
• 2 tbsp unsalted butter, divided — 1 tbsp for dish, 1 tbsp for sauté (use high-fat European style)
• 1 tbsp olive oil — for sautéing onion (choose extra-virgin)
• 1 small yellow onion, thinly sliced — cook until lightly golden (look for sweet, firm bulbs)
• 3 cloves garlic, minced — add briefly to avoid bitterness (use fresh, not jarred)
• 360 ml heavy cream — rich base for gratin (check for minimal additives)
• 120 ml whole milk — lightens the cream mixture (use grass-fed if available)
• 1 tsp Dijon mustard — for subtle tang (smooth, traditional style)
• 1 tsp kosher salt — plus more to taste (Diamond Crystal preferred)
• 0.5 tsp black pepper, freshly ground — balances richness (grind coarse)
• 0.25 tsp ground nutmeg — classic gratin spice (freshly grated if possible)
• 170 g Gruyère cheese, shredded — melts creamy and nutty (smoked optional for depth)
• 50 g Parmesan cheese, grated — savory top finish (Parmigiano-Reggiano if possible)
• 2 tbsp fresh thyme leaves, chopped — infuses the cream (use tender stems only)
• 0.25 cup panko breadcrumbs, optional — extra crunch on top (toast lightly if desired)
• 1 tbsp parsley, chopped — bright garnish (flat-leaf for best flavor)

Step-by-Step Method

Preheat & Prep the Dish

Preheat the oven to 400°F. Butter a 9×13-inch baking dish with 1 tablespoon butter. Gather all equipment and ingredients. Trim and halve Brussels sprouts, and thinly slice potatoes about 1/8 inch thick.

Thinly slice the onion and mince the garlic. Shred Gruyère, grate Parmesan, and chop thyme and parsley. Set everything within reach.

Blanch the Brussels Sprouts

Bring a large pot of salted water to a boil. Add Brussels sprouts and blanch for 3 minutes to soften and tame bitterness.

Drain immediately and pat dry thoroughly to prevent watering down the sauce. Set aside to cool slightly. This step makes certain the sprouts cook evenly with the potatoes and keep a pleasant bite.

Sauté the Onions & Garlic

Heat olive oil and the remaining tablespoon of butter in a skillet over medium heat. Add the sliced onion and cook until soft and lightly golden, about 6 minutes.

Stir occasionally for even color. Add minced garlic and cook for 30 seconds until fragrant. Remove from heat to prevent burning and set aside.

Whisk the Cream Mixture

In a mixing bowl, whisk together heavy cream, whole milk, Dijon mustard, kosher salt, black pepper, nutmeg, and chopped thyme. Whisk until smooth and well combined.

Taste and adjust salt as needed. Keep the mixture nearby for quick assembly. The mustard and nutmeg add depth while balancing richness.

Layer the Potatoes & Season

Arrange half the sliced potatoes in the prepared baking dish, slightly overlapping in shingled rows. Season lightly with a pinch of salt and freshly ground black pepper.

Keep layers even for consistent cooking. The thin slices help the potatoes cook through and absorb the seasoned cream mixture.

Add Brussels Sprouts, Onions & Cheese

Scatter half the blanched Brussels sprouts and half the sautéed onions over the potatoes. Sprinkle with half the shredded Gruyère, distributing evenly.

Repeat the layers with remaining potatoes, Brussels sprouts, onions, and Gruyère. Keep the top relatively flat to promote even browning and a cohesive gratin.

Pour the Cream & Settle

Pour the cream mixture evenly over the layered vegetables, aiming to cover all surfaces. Tap the dish gently on the counter to help the liquid settle around the layers.

Make sure the corners are filled. This step prevents dry pockets and creates a silky, cohesive sauce as the gratin bakes.

Top with Parmesan (and Panko)

Combine grated Parmesan with panko breadcrumbs if using. Sprinkle the mixture evenly over the top to create a crisp, golden crust.

Use your fingers for even distribution. The Parmesan adds savory depth, while panko increases crunch. Skip panko if you prefer a purely cheesy finish.

Bake Covered, Then Uncovered

Cover the dish loosely with aluminum foil. Bake for 25 minutes to steam and tenderize the layers.

Remove the foil and continue baking 20 to 25 minutes more, until the top is deeply golden and the potatoes are tender when pierced. Watch the edges for bubbling and even browning.

Rest, Garnish & Serve

Let the gratin rest for 10 minutes to set and thicken. Sprinkle with chopped parsley for freshness and color.

Slice into squares and serve warm. Pair with a bright salad or roasted protein. Leftovers reheat well at 350°F covered for 15 to 20 minutes or in short microwave bursts.

Ingredient Swaps

• Dairy-free: Use coconut cream or cashew cream plus unsweetened oat milk; swap Gruyère/Parmesan with dairy-free mozzarella/parmesan; use olive oil instead of butter.
• Lighter: Replace heavy cream with half-and-half or evaporated milk; use part-skim mozzarella or sharp white cheddar instead of Gruyère.
• Gluten-free: Skip panko or use GF breadcrumbs or crushed GF crackers.
• Budget: Sub cheddar or Monterey Jack for Gruyère; skip Parmesan or use a smaller amount; yellow potatoes or russets for Yukon Golds.
• Regional/availability: Use broccoli, cauliflower, or cabbage wedges if Brussels sprouts are scarce; use shallots or leeks for onion; swap thyme with rosemary, sage, or Italian seasoning.
• Flavor boosts: Add bacon or pancetta (or smoked paprika for vegetarian smoky notes); Dijon can be swapped with grainy mustard.

You Must Know

Doneness • If the top is browned but the center feels firm when pierced, tent with foil and continue 8–12 minutes until a knife slides through with almost no resistance and bubbles show at the edges.

Troubleshoot • If the sauce looks thin after cooking, let the dish stand uncovered 12–15 minutes; starches thicken as it cools slightly, turning loose cream into a silky nappe that clings to a spoon.

Flavor Boost • For deeper savor, toss the sliced potatoes with 1 teaspoon kosher salt and 1/2 teaspoon black pepper 15 minutes ahead; mild surface curing draws moisture, concentrates flavor, and seasons more evenly.

Swap • For a lighter profile, use 1 cup heavy cream + 1 cup low‑fat milk; expect an extra 10–15 minutes covered time until the center visibly bubbles and the top reaches a rich golden color.

Scale • For 8–10 servings in a 12×8 or 3‑quart dish, increase all ingredients by 1.5x and extend covered time by 10–15 minutes; visual cue is uniform bubbling across the middle before uncovering.

Serving Tips

• Serve with roast chicken, seared pork chops, or baked salmon for balance.
• Add brightness: lemon-dressed arugula or apple-fennel slaw on the side.
• Plate in warm shallow bowls; spoon extra cheesy cream over top.
• Top with crisp bacon bits or toasted almonds for crunch and savor.
• Finish with fresh thyme leaves and a squeeze of lemon to cut richness.

Storage & Make-Ahead

Refrigerate leftovers in an airtight container 3–4 days.

Reheat covered at 350°F for 15–20 minutes or in short microwave bursts.

Assemble up to 24 hours ahead, cover, and chill.

Add 10–15 minutes to the covered bake time.

Freezing isn’t ideal for creamy gratins, but tightly wrapped portions freeze up to 2 months.

Reheating

Reheat gently.

Cover and warm in a 350°F oven for 15–20 minutes.

For microwave, heat covered in short bursts, stirring.

On stovetop, use low heat with a splash of cream, stirring until hot.

Midwestern Church Potlucks

Often, I carry this Brussels Sprouts Potato Gratin Casserole into a Midwestern church basement and watch heads turn as the lid lifts and the aroma of cream, Gruyère, and thyme mingles with coffee and casseroles.

You know the scene: long folding tables, crockpots humming, kids weaving through pews. I slide the pan beside ham buns and Jell-O salads, then claim an outlet to keep it warm.

Here’s how I make it potluck-perfect:

1. Bake covered, then uncover to brown; rest 10 minutes so slices hold.
2. Transport in the same 9×13; wrap tight with foil and a towel.
3. Label with ingredients—Brussels sprouts, potatoes, dairy—clear and friendly.
4. Set out a firm serving spoon; cut neat squares for easy scooping.

Expect emptied corners and quick recipe requests.

Final Thoughts

Ready to dig in? Give this cozy gratin a try as written, or make it your own with smoked cheese, a pinch of red pepper flakes, or your favorite herbs—then tell me how you liked it!

Frequently Asked Questions

Can I Make This Gluten-Free Without Sacrificing the Crispy Topping?

Yes—you can. I swap panko with crushed gluten-free crackers or puffed rice, toss with grated Parmesan, olive oil, and thyme. Scatter generously; it bakes shatteringly crisp, nutty, and golden without crumbs turning soggy. You won’t miss gluten.

What Wine Pairs Best With This Gratin for Dinner?

I’d pour a crisp, nutty white: Chardonnay (lightly oaked) or Grüner Veltliner. Their acidity cuts richness, subtle spice flatters thyme and nutmeg. If you prefer red, I’d choose Pinot Noir—silky, bright, and mushroomy. Cheers!

How Do I Scale the Recipe for a Crowd of 12?

Double everything for 12. I’d use two 9×13 dishes, keep layers thin, and rotate pans halfway. Add 10–15 minutes to covered bake if starting cold. Rest 10 minutes; garnish generously—cheesy, creamy edges will sing.

Can I Freeze the Unbaked Casserole Successfully?

Yes—you can freeze it unbaked. I cool layers completely, use heavy cream, wrap tightly, and freeze up to 2 months. Bake from frozen, covered, at 400°F about 45–55 minutes, then uncover until bubbling, golden, and fragrant.

What’s the Best Way to Prevent Curdling in the Cream Sauce?

Keep dairy from curdling by gently heating, using full-fat cream, and avoiding a rapid boil. I whisk in Dijon and seasonings first, temper with warm cream, salt last, and bake covered initially—silky, fragrant sauce every time.